I. Field of the Invention
The present invention relates generally to lithographic semi-conductor processing. More particularly, the present invention relates to a method for forming a diamond stencil mask for use in lithographic patterning of semi-conductors devices and circuitry.
II. Description of the Prior Art
The diminution of structures produced on semi-conductors has steadily progressed in recent years. Traditionally, photolithographic processes have been used to create structures on semiconductors. These traditional photlithographic processes employ optical processes to create an xe2x80x9cimagexe2x80x9d of the structures to be produced on the semi-conductor substrate. These optical xe2x80x9cimagesxe2x80x9d are captured in methods similar to photographic development.
The reduced size of structures produced on semi-conductors, however, increasingly requires lithographic methods other than traditional photolithography. To this end, new lithographic techniques have been developed. For example, electron beam lithography uses electrons, rather than the photons of traditional photolithography, to produce a lithographic image on a substrate. The use of electrons, rather than photons, allows for the creation of smaller images, as the shorter wavelength of electrons allows superior resolution than even very high energy photons.
Traditional lithography utilizes an optical mask to form the image on the substrate. An optical mask includes sections that are transparent to the wavelength of light used in the photolithographic process. The image is formed on the substrate by the selective transmission of light through the optical mask to the substrate. Stencil masks, commonly referred to simply as stencils, have been developed to perform the same function in ion beam and electron beam lithography as an optical mask performs in photolithography. A stencil is the physical structure that selectively blocks radiation such as electrons or ions from transmission from a source to the substrate. Stencils, unlike optical masks, use openings or perforations extending through the stencil rather than a transparent section of material to allow for the selective transmission of radiation. The reason for this difference is that the radiation used with stencils, such as ions and electrons, cannot pass through a xe2x80x9ctransparentxe2x80x9d material unless that material is so thin as to be impractical to use in a lithographic process. Hence, openings in the stencil correspond to areas of the substrate that are exposed, while opaque portions of the stencil correspond to portions of the substrate that do not receive radiation. This selective blocking of radiation creates an image upon the substrate that is subsequently converted into the electronic structures on the semi-conductor.
Traditionally, stencils for use in conventional photolithography and electron beam lithography have been constructed of silicon. Typically, the stencils themselves are fabricated using lithographic techniques. In the past, silicon has been an acceptable material for use as a stencil because it is readily available, fairly durable, and undergoes limited expansion/distortion during use for traditional lithography. Other materials, however, may prove superior to silicon for use as a stencil material, particularly for use with electron beam or ion beam lithography.
Electron beam lithography presents unique challenges in the production and use of stencils. The creation of a stencil comprises a trade off between the stencil fabrication process and stencil durability. In general, a thinner stencil may be fabricated more easily than a thicker stencil. A thicker stencil will typically provide advantages of rigidity and durability over a thinner stencil, but generally will also be more difficult to fabricate. Designing a silicon stencil is thus a trade off between ease of fabrication and utility.
The use of a silicon stencil with electron beam lithography poses problems of heat effects. Electron beam lithography creates more heat in the mask than conventional photolithography. The heating of the stencil causes it to expand, which can lead to the distortion of image placement. There are two properties of a material that determine how great a problem the heating of the stencil is. One characteristic of a material relevant to its performance as a stencil is the thermal conductivity of the material used to create the stencil, which describes how effectively the stencil transmits heat. While thermal conductivity is a general characteristic of materials, this characteristic is critical in stencil design because a material will heat in response to exposure to an electron beam or an ion beam. The second property of a material relevant to its performance as a stencil is its coefficient of expansion, which determines how much the material expands due to heating.
Silicon""s thermal conductivity and its coefficient of expansion are both acceptable for use in current electron beam lithography, but may not be acceptable for use in electron beam lithography as the critical dimensions of printed images continue to shrink. The use of a silicon stencil in electron beam lithography can distort image placement due to the heating and expansion of the stencil. This distortion of the image placement poses a relatively greater problem as image size reduces. Of course, the creation of reduced images is the impetus for using electron beam lithography. Accordingly, other materials are needed to permit the effective use of electron beam lithography in semi-conductor fabrication.
One material that addresses the concerns of reduced thickness as well as concerns regarding the heating of the stencil is diamond. The thermal coefficient for diamond is considerably greater than for silicon, which correlates with less heating and therefore less expansion in a diamond stencil as compared to a silicon stencil. Specifically, diamond""s thermal conductivity is more than six times greater than the thermal conductivity of silicon. Silicon""s thermal conductivity is 156 watts/(meter)(xc2x0 Celsius), while diamond""s thermal conductivity is approximately 1000 watts/(meter)(xc2x0 Celsius).
As a result, a diamond stencil used in electron beam lithography does not heat as much as a silicon stencil would, resulting in less expansion of the diamond stencil. Diamond is also well known for its strength and durability. Young""s modulus for diamond, which describes its stiffness, is 900 GPa, compared to a Young""s modulus of 160 GPa for silicon. This strength and durability allows a stencil made from a diamond to be thin relative to a traditional silicon stencil without sacrificing durability. This enhanced stiffness also serves to reduce image displacement caused by physical distortions of the stencil. Accordingly, diamond makes an excellent choice for a stencil material for use with electron beam lithography.
The use of diamond stencils for electron beam lithography, while providing several advantages, also creates certain difficulties, particularly with regard to the fabrication of diamond stencils. One challenge in using diamond as a stencil material is the difficulty in forming a high quality diamond layer upon different starting materials. Diamond is particularly difficult to grow over an oxide layer. However, an oxide layer underlying a diamond layer is useful, as its use as an etch stop provides improved etch profile control when the diamond layer is etched.
Another obstacle to the creation of diamond stencils for use in lithography has been the lack of a technique to allow for sufficient profile control in etches of the diamond stencil. The present invention overcomes these impediments to the use and creation of diamond stencils by using a nucleation layer upon which a diamond film is grown and providing an etch stop layer for use in the formation of a diamond stencil.
The present invention comprises a method for forming a diamond stencil for use in lithography, particularly electron beam lithography. In accordance with the present invention, a thin diamond layer is formed on a substrate using a nucleation layer to assist in growing a diamond film. In accordance with the preferred embodiment of the present invention, the substrate used includes an etch stop layer, such as a buried oxide layer, below the nucleation layer. This etch stop layer later serves as an etch stop in both a backside reactive ion etch used to expose the diamond membrane and a front side etch of the diamond to form stencil openings. This use of an etch stop allows for better control of the etch processes, and therefore allows the formation of a more well defined stencil.
FIGS. 1-8 illustrates substrates at various steps in the process of forming a diamond stencil in accordance with the present invention; and
FIG. 9 is a flow diagram illustrating a method for forming a diamond stencil mask in accordance with the present invention.